Process for the production of ceramic products



Patented May lid,

WILHELM vnasnornn, or BAMBERG, enamANY.

PROCESS FOR PRODUCTION OF CERAMIC PRODUCTS.

No IDrawingn Application filed September 26, 1923, Serial No. 665,009, and in Germany September27, 1922. v

invention relates to the production of ceramic products'and more particularly to themanufacture of sintered clay articles in can eflicient' and economical manner,

In the production of ceramic articles it has heretofore beeniproposed to effect cementi-ng or silicating of clays before molding thereof by the addition thereto ofzwate'r soluble fluorides, the fluorides rendering the 1o molded articles capable of setting,

It has been found that with certain clays the reaction due to the presence of a water 'soluble fluoride is "very weak, proceeding very slowly and only to a limited extent,-

115 while with some clays the reaction does not (occur at all. Numerous experimentsjha've shown that the purer the clay, the more difficult it is to produce a cementing or silicating of the clay resulting from the reaction .20 of the fluoride, The reason for such inactivity of water soluble fluorideson certain clays appears to be that the fluorides do not act directly upon the clay but only through the intermediary of the quartz or-silica con 26 tained' in the clay; Gonsequently' in the processing of cla s which are devoid or contain an insuficient amount ofsilica, the addition of a fluoride is with little or no effect since the desired reactioncan not prog0 ceed to a suflicient extent.

According tothe present invention, the

free silica which may be present in the clay is increased in order to-ensure the actionof the fluoride in bringing about the desired at silicating or cementing. Where the clay is low in or free of quartz, silica is added thereto prior to molding in an amount sufiicient to cause fluxing of the fluoride so that on sintering a binder will be formed and a to glaze will be produced on the surface of the article.

' It hasbeen found that with addition of a comparatively low percentage of soluble fluoride to the clay, only a part'offt'he added 7 silica reacts with the fluoridewhile the. re-

maining silica is acted upon bylnascenthydrofluoricacid set free inthe reaction between the soluble fluoride and the first part -'of the silica referred to. The silica then is acted upon both bythe soluble fluoride and liberated hydrofluoric acid.

The pr mary reaction betweenthe silica" obtained accor previously known processes and fluoride may-be expressed by the following equation:

and, as stated above when only a'small per-/ centage' of fluorideis added to the clay so that there remains an excess of silica, the excess silica is acted upon in a secondary reaction by the hydrofluoric acid liberated in the first reaction according to the above equation. The secondary reaction probably proceeds according to either one or possibly both of the following equations: 4 H

7 After treatment as above described, the mass is molded in any suitable manner and 7 fired, the firing requiring only a comparatively short time "owing to the addition of the soluble fluoride and it may be carried out at a comparatively low degree of heat, for example, between 600 and 900 C. The sintering of the mass is eliected at a lower temper-attire than is required for the clay alone. i

By. the addition of a higher percentage of fluoride to the mass, a bright glaze-like so layer is'formed on the surface of the article during the firing, the fluorideserying as flux and binder at the elevated temperature producing sintering of the clay. It will be seen then that since by the use of a higher 8 percentage of fluoride a glaze isproduced on the surface of the article, it is unnecessary to' resort to additional treatment. for eflecting a glaze, such treatment being eliminated by the present invention. The glaze ing to the present invention is much more intimately associated with the core of the article than a glaze obtained by By treatment in the foregoing manner, however, 1t is possible that evenat a moderate temperature a total softening or fusion of the mass may occur which must be avoided in order to preserve the shape of:

.the molded article. To prevent such softening orifusion, according to the present invention', a calcium salt .is added to the charge,-'the calcium salt cbmbining with the water soluble fluoride to form compounds which prevent the fluorides from leading to complete vitrification of the body. The addition of calcium salts together with the soluble fluorides presents a further advantage in the color of the article after baking. Such clays which normally turn to a reddish color on baking are found to be white in color after baking when an addition of calcium salt has been made to the charge. Clays which are colored black by reason of contact of the charge with iron, for example the iron contained in the stirring mechanism, are effected in such a manner by the calcium salts that the baked article does not show iron spots.

The proportions of materials to be used according to this invention vary greatly with the chemical compositionot the clay used, and with the amounts of quartz and flux contained therein. Clays found in different localities vary greatly as to their chemical composition and it is therefor impossible to state any one set of proportions or any limits therefor, since proportions suitable for one specific clay may not be as suitable or not at all-suitable for another specific clay.

My invention isgeneric with respect to practically all clays, and it is accordingly impossible to state the most preferred proportions; and in fact any statement of proportions for one type of clay may not aid in explaining my invention with respect to other types of clay.

For example, with some clays I canuse 27% of clay, 60%. of silieic acid (quartz), 10% of a soluble fluoride, and 3% of a calcium salt. These proportions will have to be varied greatly, not only according to the chemical composition of the clay, but according to how refractory the clay happens to be. If the clay be somewhat refractory, then the calcium salt might perhaps be reduced to 1%, the quartz to 50%, or if such quartz is naturally present, then much less would need to be added.

A somewhat generally applicable formula might perha s be: 50% to 60% of-silicic acid (quartzg, 1% to 10%. ofa soluble fluoride, 1% to 3% of a calcium salt, the' balance of clay. K

i It is to be understood, however, that while I have stated this'formula to be generally applicable it may need to be varied, according to the type of clay used; but'a more eneral formula cannot be stated. I, thereore, do not wish such formula or the proportions given to limit the scope of my invention. I

According to prior processes ofpreparing ceramic masses, the charge was usually subjected to a wet mixing in' tossing tubs and in washing plants with the resultant inconvenience that excess water had to be pressed out in expensive filter presses. Such moonveniences are eliminated by the process of the present invention. i

According to my improved process, the clay is first crushed, finely sifted in any convenient manner, and mixed in a dry state with quartz and a water soluble fluoride, the amounts of quartz and fluoride depending upon and varying with the particular clay being processed. After the mixing of the charge, there is mixed therewith only the minimum amount of water which is necessary for the molding, stamping, or pressing of the raw mass. in this manner the necessity of subsequent homogenization is eliminated as the mass is sufficiently homogenized during the mixing. After the mixing stage the. material is directly moldable and pressable and after molding or pressing, it is air dried in any suitable apparatus whereby the first stage of the process is completed.

The first stage of the manufacture presents essential advantages over the production of earthenware and porcelain as heretofore known. The entire plant consisting of tossing tubs, washing channels, homogenizing apparatus, and filter presses with the accompanying expensive consumption of filter-cloth. is no longer necessary. F urther, the crushing and sifting of the charge preparatory to the molding requires only about a half hour while each of the mixing steps, that is the dry mixing and the mixing with water, requires but about fifteen minutes so that not more than one and a half hours are consumed from the beginning of the manufacture to the production of the molded article. This means an economy of several days in comparison with the pro duction of the molded article according to the usual manufacture of earthenware and porcelain, a

The second stage of the manufacture is quitesimple. The molded article, after air dryingfor about eight days, is brought directly into the lire and can be readily glazed and removed at a comparatively lowtemperature of about 900 C. The article thus produced is harder than porcelain and earthenware as prepared according to prior processes.

The second stage of the manufacture also presents important advantages over known processes. A separate glaze-baking of the article is eliminated, the article being heated but once and then at a comparatively low temperature. Further, the entire plant reuired for putting on a glaze is omitted t ereby avoiding all opportunities for loss which result from the repeated charging anddischarging of the oven, etc.

It will therefore be seen that the production of ceramic articles according to the process of the present invention affords many economic advantages consisting in facilitating and accelerating the production,

econom in time and fuel, and in the avoidin of oss by breakage and the like.

aving-describedmy invention, what I claim is:- j

1. The rocess of making sintered clay articles W ich comprises adding a soluble for the clay alone.

2. The process of making sintered clay articles which comprises adding a soluble fluoride to the clay, increasing the-free silica which may be present in the clay to an amount sufiicient to cause said fluoride to flux therewith, the amount of fluoride prescut and vthe corresponding amount of Silica being such that on sintering a binder will be formed and a glaze will be formed on the surface of the article, and sintering at a lower temperature than is required for the clay alone.

3. The process of making sintered clay articles which comprises adding a soluble fluoride to the clay, increasing the free silica which may be present in the clay to an amount sufficient to cause said fluoride to flux there withand thereby to act as a binder on sintering, adding a calcium salt'to the mass to prevent fusion, and'sintering at a lower temperature than is required for the clay alone.

4. The process of making sintered clay articles which comprises adding a soluble fluoride to the clay, increasing the free silica which may be present in the clay to an amount sufiicient to cause said fluoride to flux therewith, the amount of fluoride pres ent and the corresponding amount of silica being such thaton sintering a binder will be formed and a glaze will be formed on the- Surface of the article, adding a calcium -salt. to the mass to prevent fusion, and sintering at a lower temperature than is required for the clay. alone. a

5. The process which comprises crushing ,clay, finely sifting it, adding a soluble fluoride thereto, increasing the free silica which may be present in the clay to an amountfsufficient to ensure the silicating or cementing action of said fluoride and thereby to act as a binder on sintering, the minimum amount of water required for molding the mass being added, mixing in a dry mixing machine, molding, air drying the molded articles, and sintering at a lower temperature than is re quired for the'clay alone.

In testimony whereof, I have signed my name to this specification at Munich, Germany, this llth day of September, 1923.

v WILHELM VERSHOFEN. 

